You will need to enable the interrupt for CM0+ in the Design Wide Resoruces. In the Workspace explorer, under "Design Wide Resources" double-click "Interrupts". In that tab, make sure WDTIsr is enabled (checked) for M0+ (or rather the core you want the interrupt to be mapped to). If you build your project after this, the WDTIsr_cfg will be defined for CM0+.
I guess the above should remove the error.
Meenakshi Sundaram R